1. Field of the Invention
The present invention relates to a servo control method for a hard disc drive, and more particularly, to a sequential seek method to move a head of a hard disc drive from a track to an adjacent track, a hard disc drive using the same, and a computer readable recording medium storing a program to execute the same.
2. Description of the Related Art
A hard disc drive (HDD) is a device for storing information on discs. Generally, in a HDD, information is written on concentric tracks on a surface of a disc. The disc is mounted on a spindle motor. The surface of the disc is accessed by a read/write head, which is mounted on an actuator arm being rotated by a voice coil motor (VCM). To rotate the actuator, the VCM is controlled by a servo circuit. The read/write head detects a magnetic field on a surface of the disc to read information recorded thereon. Meanwhile, the read/write head generates a magnetic field to write information to the surface of the disc. The magnetic field generated by the read/write head magnetizes the surface of disc in accordance with the desired information to write. In read/write operations, the servo circuit controls a movement of the read/write head by providing the VCM with a VCM driving current.
According to a tendency of high storage capacity, a track pitch of a HDD has become narrower. As the track pitch becomes narrower, the interference between tracks becomes more prominent. Although this interference, called an adjacent track erase (ATE), is mainly influenced by a track pitch, there are other factors that affect it such as a head width, a frequency characteristic of a head, a head gap, a location on the disc, etc. Since the location on the disc is more important than the other factors, it is necessary to vary the track pitch accordingly.
To vary a track pitch according to the location on the disc, a flexible data track (FDT) is used. A disc has a plurality of tracks and every track includes servo sectors and data sectors. In general, one or more data sectors are placed between servo sectors in an embedded servo. By the FDT, a track pitch of a servo sector and that of a data sector are essentially different from each other and the difference there between varies according to the location on the disc.
FIG. 1 illustrates an arrangement of servo sectors and data sectors according to the FDT. According to the FDT, a track pitch of a servo sector and that of a data sector becomes different from each other as shown in FIG. 1. Generally, a surface of a disc of a HDD is segmented into a plurality of areas, so called zones, and a data track pitch varies according to the respective zones. In addition, the further out the zone on the disc is, the larger the data track pitch is. The data track pitch for a respective zone is contained in a zone map table as one of the parameters for specifying the structure of zones.
Such a variance of the data track pitch may sometimes obstruct track seek operations and, in particular, a track seek operation in the outer area of the disc.
FIG. 2 illustrates a flow chart of a conventional track seek method. Referring to FIG. 2, it can be seen that the conventional track seek method is based on the seek length, which is calculated as a difference between a current cylinder number and a target cylinder number. In a HDD, tracks in a vertical direction, called a cylinder, all have the same track number, and thus, sometimes a track or a track address is referred to as a cylinder or a cylinder number.
Referring to FIG. 2, in operation S202, a seek length sk_len is calculated as a difference between a current cylinder number current_cyl and a target cylinder number target_cyl.
In operation S204, a seek time sk_time, which is a time for moving a head as much as the seek length sk_len, is calculated.
In operation S206, seek trajectories to specify behaviors of the head and a VCM driving current during a track seek operation are calculated by using the seek length sk_len and the seek time sk_time. Seek trajectories includes a position trajectory, a velocity trajectory, and a current trajectory.
In operation S208, a VCM is driven to move the head based on the seek trajectories. The head reads servo sectors while it is being moved by the VCM. By using servo information contained in servo sectors, states of the head, for example, a position of the head, a velocity of the head, an acceleration of the head, etc., are estimated and compared with those of a certain trajectory. The results of the comparison are forwardly fed to control the VCM driving current.
In operation S210, it is checked whether the head arrived at the target cylinder target_cyl. If it is determined that the head did not arrive at the target cylinder target_cyl, then the method returns to operation S208. Otherwise, the track seek operation is terminated.
As can be seen in FIG. 2, in a conventional servo control method, the seek length sk_len is calculated based on the cylinder numbers of a current cylinder current_cyl and a target cylinder target_cyl. However, by this characteristic of the conventional servo control method, sometimes, a track seek operation is obstructed in a HDD adopting a FDT.
FIG. 3 illustrates different cases for a track seek operation over adjacent data tracks (hereinafter a sequential seek). Referring to FIG. 3, a difference between sector tracks in one case, denoted by {circle around (1)} (case 1), is 1, whereas in the other case, denoted by {circle around (2)} (case 2), is 2, despite the fact that the differences between data tracks are all equal to 1. This means that the seek time sk_time is longer in case 2 than in case 1 for a sequential seek, which negatively affects the performance of a HDD.
A specification of a HDD specifies the time of the sequential seek. As is well known, tracks of a HDD are formed in concentric circles. This means that the head ought to move to a next track to read/write subsequent data when the subsequent data is allocated to the next track.
To increase the rate of data transmission, a movement of the head ought to be synchronized with the transmission of data, and thus, a time to move the head from a track to next track is strictly specified.
Referring to FIG. 3 again, synchronization can be achieved in case 1, whereas it cannot be achieved in case 2. If a data transmission operation is not synchronized with a head movement, the performance of a HDD is lowered. Thus a track seek operation cannot be done in a requested sequential seek time, and thereby a data transmission operation has to be delayed until a target data sector is relocated under the head after one more rotation of a disc.
Further, referring to FIG. 3, the possibility not to achieve synchronization in a sequential seek is increased relative to the increase of the difference between the track pitch of a servo track and that of a data track. Accordingly, the possibility of lowering the performance of the HDD is increased when a sequential seek takes place in an outer area of the disc.